Invasive Pathogens At Alpine Treeline: Consequences for Treeline Dynamics

Abstract

The potential impact of invasive forest pathogens on alpine treeline dynamics has not previously been considered. Whitebark pine (Pinus albicaulis), a foundation and keystone species of subalpine forests and major component of alpine treeline in the northern Rocky Mountains of the United States and southern Canada, is infected nearly range-wide by the exotic pathogen Cronartium ribicola, which causes white pine blister rust. A major component of treeline in the northern Rocky Mountains, whitebark pine initiates tree islands on the eastern slope in northwestern Montana more than any other conifer species. Blister rust infects whitebark pine throughout the region, and both infection and mortality are already evident at treeline. We discuss the cascading ecological effects of the loss of treeline whitebark pine and expected changes in landscape vegetation patterns. Potential implications of the loss of whitebark pine for northwestern Montana treelines are examined in the context of climate change within a conceptual model. We speculate that exotic pathogens could potentially confound predictions of treeline responses to global warming in many geographic regions.

Keywords:

• alpine treeline
• whitebark pine
• white pine blister rust